The printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a versatile thermal processless printing plate material has been noticed which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent, and treated in the same manner as in PS plates.
In a printing plate material comprising a metal substrate, particularly a grained aluminum substrate, and provided thereon, a component layer comprising a functional layer on which an image is recorded by imagewise exposure of infrared laser, sensitivity for image formation varies due to a balance between quantity of heat generated during the exposure by a light heat conversion material, contained in the functional layer or another layer component layer, and heat diffusion onto the substrate. The balance is greatly affected by the thickness of the component layer comprising a functional layer.
Since the surface of the grained aluminum substrate ordinarily has a convexoconcave structure from sub microns to scores of microns, a component layer provided on the substrate has a microscopic thickness distribution corresponding to the convexoconcave structure of the substrate, and is considered to have sensitivity differing due to the microscopic thickness distribution. Therefore, when a printing plate material having a broad layer thickness distribution is exposed, exposure may be excessive in some portions of the material but insufficient in other portions of the material, which may result in lowering of latitude of exposure.
In a printing plate material comprising an grained aluminum plate, which is imagewise exposed and developed to form an image, developability is affected by the convexoconcave structure of the grained aluminum plate surface, and particularly, a printing plate material comprising an aluminum plate having deep pits on the surface and a component layer provided thereon is difficult to develop on account of the component layer deeply incorporated in the pits. In contrast, a printing plate material comprising a component layer prepared so as to be easily removed on development, although incorporated in the deep pits, results in problem in that strength of the layer is lowered at image portions. Accordingly, such a printing plate material as aforementioned has a tendency to reduce latitude for development.
In order to solve the above problem, a printing plate material is disclosed in for example, Japanese Patent O.P.I. Publication No. 2002-99092, which comprises a surface-roughened aluminum plate and a recording layer provided thereon, the plate having a 80° glossiness of not more than 30, and having ten or less pits with an opening width of not less than 10 μm per a length of 1 mm or ten or less pits with a maximum depth in the direction perpendicular to the width of not less than 1.7 μm per a length of 1 mm, and the recording layer containing an infrared absorbing agent, and a water-insoluble and aqueous alkali solution-soluble polymer, which increases alkali solubility after infrared laser exposure. It is considered that the aluminum plate having such a surface configuration as provided above can reduce a residual layer after development. However, such a provision cannot be regarded as meeting necessary and sufficient conditions for a substrate surface configuration. It is extremely insufficient for a surface configuration of an aluminum plate used in a printing plate material comprising a water-developable component layer, particularly a processless printing plate of development on press type.
So far, detailed study has not been made on a minute surface configuration of a grained aluminum plate suitable for a processless printing plate material.